This invention relates to an internal combustion engine and, more particularly, to internal combustion engines with greatly reduced emission of pollutants.
As the emission from the automotive internal combustion engine and their substantial contribution to air pollution have become known, there has been a great deal of interest in the goal of a virtually pollution-free automobile. This has led to the adoption of increasingly more stringent governmental specifications and requirements which have given great impetus to the search for a satisfactory solution. A number of alternative power sources, such as the steam engine and battery power, have been considered, but these alternatives have a great number of practical drawbacks. It has also been proposed that expensive, bulky, and complex exhaust thermal reactors and catalytic converters be added on to existing engines. However, these solutions, which aim at cleaning up the combustion products after they are created, are, at best, no more than stopgap measures. In terms of cost, maintainability, reliability, performance, and economy, these measures would, effectively, move the internal combustion engine back in time to the turn of the century when the automobile was an expensive and complex device suitable for operation only by the rich.
A more promising approach involves an investigation of the combustion process and the activities within the combustion chamber itself and consideration of the causes of the principal pollutants which appear in the engine exhaust and crankcase blowby. Unburned hydrocarbon droplets tend to collect on cylinder walls and due to the relative coolness of the wall are not completely combusted. Likewise, unburned hydrocarbons are forced into the crevice between the cylinder wall and the piston above the first ring and are not completely burned. It has been found that the emission of unburned hydrocarbons is at a maximum for rich mixtures and at lower engine speeds and falls off for lean mixtures and at higher engine speeds. Carbon monoxide is another emission product which results from imcomplete combustion. The production of carbon monoxide is also at a maximum at lower engine speeds and for rich mixtures, falling to a minimum for lean mixtures and at higher speeds. Oxides of nitrogen are formed at the higher engine temperatures which are characteristic of high engine speeds and vehicle loading. As a result, the emission of nitrogen oxides tends to be negligible at lower engine speeds and loading and increases sharply at higher engine speeds and loading.
Before considering prior art proposals for modifying the combustion process to reduce these pollutants, it is well be recognize that any solution, if it is to meet with ready acceptance, should accommodate to certain practice realities. Since the great bulk of the automotive vehicles in use today fall within the category of normal family cars, any solution to the problem should meet the demands of vehicle of this type. The system should be suitable for mass-production and should not add appreciably to the cost of the vehicle. Since family vehicles are frequently maintained by relatively unskilled mechanics, it is essential that the system adopted should be easily maintained at normal maintenance intervals. Moreover, the system should be somewhat fail safe -- if the system fails in whole or in part, the engine should not be destroyed as a consequence. Engines presently provided in family cars often last as long as 100,000 miles without the need of major overhaul. Therefore, the components of the system providing a pollution-free engine should have a mean life of at least 100,000 miles. In addition, the system adopted should not significantly affect the operating economy of the family automobile. To summarize, a pollution-free engine must not make the normal family car appreciably more expensive or difficult to maintain or less reliable or economical to operate than present day vehicles of this type.
These criteria must be kept in mind when we examine existing proposals for the modification of the combustion process itself. One prior art approach involves the use of a fuel injection stratified charge engine of the types proposed by Sir Harry Ricardo's group and FM M.A.N. These engines, however, which are, in effect, part otto cycle and part diesel engines, are subject to the drawbacks that they are noisy and expensive. The Texaco Combustion Process as described in SAE Transactions, 1961, pp.120-134, is a promising candidate of the stratified charge type. However, the Texaco Combustion Process requires very precise timing and control functions to reach its low emission potential, and the manufacturing cost may be too high for general use.
Another method of modifying the combustion process involves the injection of air into the cylinder during combustion. There have been a number of prior art proposals to this effect including, more recently, the patents of Alfred Candelise, such as U.S. Pat. No. 3,073,289. This patent disclosed experimental verification of the technique demonstrating significant reductions in carbon monoxide and unburned hydrocarbon production However, the prior art of this type, including the patents of Candelise, require the use of a mechanical compressor to provide the pressurized air and, usually, the use of some sort of mechanical timing device to time the introduction of the air to the cylinder. These add-on elements contribute greatly to the cost and complexity of the engine.
While these prior art engines are capable of reducing the emission of unburned hydrocarbons, they do not control the emission of the nitrogen oxides. For this purpose, it has been proposed to recirculate the exhaust gases. It has been found that devices which return up to 15% of the engine exhaust to the intake manifold reduce peak engine temperatures and thus reduce nitrogen oxide emissions. However, this expedient involves the addition of a recirculation pipe and requires the provision of a valve for pressure balancing, thus increasing the complexity and cost of the engine.